"The water," said John Waldman, "varies between green and shockingly green."

Perched on a thin strip of grass between a road and the water's edge, he stared intently at the surface. On a postcard fall noon it was the color of fresh spinach, the algae and silt so thick that the sun was swallowed just a few inches into the murk.

"Minnows," he said, pointing to a dappling of translucent silver-pink fish several feet from sure. "It's salty, with a sky-high pH, but it's rich and full of life nonetheless." We stood at at the mouth of a short creek between two bodies of water that elsewhere would be called ponds, but in New York City they are called Willow Lake and Meadow Lake, and are faintly miraculous. Like so much of the estuary now entombed beneath the world's eleventh-largest city, it was once a tidal marsh, and still receives the tide. This explained the salinity. The pH — nine, to be exact, same as baking soda — came from coal ash, which residents piled by the creek when Queens was still country. Builders used the ash to line the lakebeds, and it prevents the water from spreading back into the silt.

Waldman, a biology professor at nearby Queens College, hoped the minnows would attract a snakehead. The voracious intruder was found this spring in Willow Lake, likely introduced by an owner exasperated with its boundless appetite, and earlier this fall, Waldman saw a juvenile — evidence that they might have spawned.

"It's still not clear whether they are highly successful," he said. As for whether the snakeheads will eat everything in sight, fulfilling the fearsome though overstated reputations — Snakehead!! — spread by toothy tabloid covers three summers ago, when the air-breathing Asian natives were discovered in Maryland, nobody knows. "Its possible they'll overshoot the food source before reaching some sort of equilibrium," said Waldman.

Overshooting the food source, of course, is a technical way of saying, eat everything in the lakes. Worse yet, a breeding population of snakeheads would threaten any other waters to which misguided bucket-carriers could haul them. The Department of Environmental Conservation has taken control of the situation, but Waldman, who worked for twenty years at the Hudson River Foundation before taking his professorship, still comes by to look. He is a fish junky, a city boy who spent his life exploring, and later chronicling the history of, the waters around New York City — waters that, he and others discovered, are full of life, a life that is surprisingly resilient, clinging to and even thriving in niches shaped by human destruction.

Waldman crossed the street to look for snakeheads on the other side, easily vaulting a concrete divider. At fifty he is still fit and trim, with sharp blue eyes, an angler's grip and a chin of stubble that matches the white of his hair. He wore a brown jacket — herringbone, appropriately — and loose slacks, somewhat resembling the Victorian gentleman who, in a framed drawing on his office wall, carries with aplomb a man-sized fish on a stick over his back.

"The drive for life is really intense. There's an awful lot of contamination in this world," he said, speaking for a moment of life above the water as well. "They may not produce so many eggs as in a pristine environment, or grow as well, but there's still a living to be made here."

Waldman continued up the creek. The leaves had already turned, and many of the fallen had yet to lose their color; thickets of head-high phragmites lined the water's margins with green stalks and loose flaxen heads that waved back the sun's gold. We passed beneath the interleafing of the Van Wyck Highway and the Long Island Expressway, their supporting concrete columns aged by weather and almost as wide around as some of the trees that once lived in the surrounding valleys.

The water's surface vibrated from cars passing above and was boiled by thick schools of minnows that surfaced in flight from our footsteps. A chemical skein floated with lazy iridescence beside a drainage inflow where we next stopped. Littering the water was a collection of trash straight from central staging: milk crates, tires, shopping carts.

"Shopping carts are one of the major features of urban aquatic environments. Another one is the spare tire," Waldman said. Beside one bridge, he said, it is possible to walk across the water on the trash collected beneath it. "Tumbleweed," he said, pointing to a wind-blown plastic bag.

We saw no snakeheads there, or farther along at the locks that hold the lakes in. Nor did we see any other fish, though the water teems with them — sunfish, white perch, carp, catfish, the earlier-seen minnows, which are technically named killifish, and American eels, which despite their name breed in the Sargasso Sea, not far from the West Indies. These fish have survived, even thrived, amidst some of the worst pollution in the world, but now face eradication from the latest offhand transformation.

The snakehead, of course, just a few generations removed from some brackish backwater in northern China, is an unwitting victim in the whole affair, arbitrarily moved by the fate of globalization and human nature. Singling it out as an alien in an already-altered ecological balance could seem unfair — but shaping the development of nature is a power that, for better or worse, people exercise simply by existing.

"The snakehead is a gritty fish, with great survival instincts. It's hard to dislike it," John said on the walk back to the car. "But it just doesn't belong here."

Image: A satellite photograph of Meadow and Willow Lakes.

I wrote this almost five years ago (!) in school, as part of a profile on Waldman that I never did manage to get published. Which is a shame, because John is a gracious and fascinating person, whose work on New York City's aquatic ecology deserves much attention.